U.S. patent number 6,671,398 [Application Number 09/964,733] was granted by the patent office on 2003-12-30 for method and apparatus for inspection of patterned semiconductor wafers.
This patent grant is currently assigned to Applied Materials, Inc.. Invention is credited to Gilad Almogy, Silviu Reinhorn.
United States Patent |
6,671,398 |
Reinhorn , et al. |
December 30, 2003 |
Method and apparatus for inspection of patterned semiconductor
wafers
Abstract
Novel method and apparatus are disclosed for inspecting a wafer
surface to detect the presence thereon of exposed conductive
material, particularly for determining the integrity of contact
holes and vias, in semiconductor wafer manufacturing. The method
comprises the steps of irradiating a spot of the wafer surface with
a beam having a wavelength sufficiently shorter than the working
function of the metal, such as deep UV light beam, collecting the
electrons released by the irradiated wafer, generating an
electrical signal that is a function of the collected electrons,
and inspecting the signal to determine whether the contact holes or
vias within the irradiated wafer spot are open. The apparatus
comprises a vacuum chamber having therein a stage and chuck for
supporting the wafer. An illumination source generates irradiating
energy which is formed into a beam using appropriate optics so as
to obtain the desired beam spot of the wafer's surface. an electron
detector collects electrons released from the wafer surface and
sends a corresponding signal to a processor for processing the
signal to determine whether the metal at the bottom of the hole is
exposed. Optionally, the light scattered by the wafer is detected
by detectors arranged around the illuminating beam.
Inventors: |
Reinhorn; Silviu (Yavne,
IL), Almogy; Gilad (Yavne, IL) |
Assignee: |
Applied Materials, Inc. (Santa
Clara, CA)
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Family
ID: |
22533900 |
Appl.
No.: |
09/964,733 |
Filed: |
September 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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150296 |
Sep 9, 1998 |
6317514 |
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Current U.S.
Class: |
382/145; 250/305;
250/397; 250/574; 257/E21.53; 382/149 |
Current CPC
Class: |
G01N
21/95692 (20130101); H01L 22/12 (20130101) |
Current International
Class: |
G01N
21/956 (20060101); G01N 21/88 (20060101); H01L
21/66 (20060101); G06K 009/00 () |
Field of
Search: |
;382/144,145,149,151,203
;250/305,306,307,309,310,227.29,559.41,559.18,559.42,574,397
;348/126,177 ;356/237.2,237.6,369 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Detecting Photoresist Residues Using Voltage Contrast ESCA" Hook;
Physical Electronics Document No. 9801. .
"Characterization of p-n Junctions and Surface-states on Silicon
Devices by Photoemission Electron Microscopy"; M. Giesen et al.;
Applied Physics, A64, pp. 423-430 (1997)..
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Primary Examiner: Patel; Jayanti K.
Assistant Examiner: Chawan; Sheela
Attorney, Agent or Firm: Sughrue Mion LLP.
Parent Case Text
This is a Continuation of application Ser. No. 09/150,296 filed
Sep. 9, 1998 now U.S. Pat. No. 6,317,514, the disclosure of which
is incorporated herein by reference.
Claims
What is claimed is:
1. A hybrid method of inspecting a patterned semiconductor wafer,
comprising the steps of: (a) generating an illumination beam; (b)
impinging said illumination beam upon said wafer surface; (c)
performing material classification inspection of said wafer surface
using electrons released therefrom, the material classification
inspection comprising the steps of: (c1) collecting the electrons
emitted from said wafer surface due to the impingement of said
beam; (c2) generating a classification signal corresponding to the
amount of electrons collected from the wafer in step (c1); (c3)
analyzing the classification signal generated in step (c2) to
determine the presence of defects on the wafer based on the
material classification; and (d) performing scattered light
detection of the illumination beam reflected from said wafer
surface, the scattered light detection comprising the steps of:
(d1) receiving light scattered from said wafer surface onto a
plurality of detectors; and (d2) determining an existence of
surface defects based on the light received by said plurality of
detectors in step (d1).
2. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 1, wherein the illuminating beam is
impinged on a high aspect ratio wafer surface.
3. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 1, wherein the step of receiving light
scattered from said wafer surface is performed using a dark field
arrangement.
4. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 3, further comprising the step of
performing scattered light detection using a bright field
detector.
5. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 1, wherein the step of generating an
illumination beam comprises generating a deep UV light beam.
6. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 1, wherein the material classification
analyzing step comprises comparing the classification signal to a
reference signal chosen from among a database signal and a
corresponding signal produced from adjacent die or cell.
7. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 1, wherein the material classification
analyzing step comprises using the classification signal to
determine a working function of the inspected material on the wafer
surface.
8. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 1, wherein the scattered light detection
and the material classification inspection are performed
simultaneously.
9. The hybrid method of inspecting said patterned semiconductor
wafer according to claim 1, further comprising the steps of first
using the scattered light detection to sense a surface defect and
subsequently using the material classification inspection to
classify a type of material corresponding to the surface defect.
Description
FIELD OF THE INVENTION
This invention relates to method and apparatus for the inspection
of patterned semiconductor wafers and particularly for the
detection of defects that may exist in the upper layer of a wafer
which comprises structures having a high aspect ratio, such as high
aspect ratio contact holes and vias, and structures that may have
residual metal defects.
BACKGROUND OF THE INVENTION
The art comprises methods for detecting possible defects in
patterned wafers. A summary of the state of the art concerning such
detection is included in U.S. Pat. No. 5,699,447, and is
incorporated herein by reference. In said patent, an inspection
apparatus is described which comprises a table for receiving a
wafer to be inspected, a source of a laser beam, which beam scans
the surface of the wafer, and a plurality of light collectors for
collecting the light scattered from the wafer and transmitting the
scattered light to a plurality of detectors. The output of the
detectors is fed to a processor, which produces information
indicating locations on the wafer in which the presence of a defect
is suspected.
With optical scanning, it is possible to detect defects that are on
top of the upper layer of the inspected wafer, or under transparent
layers. As design rules progress, higher importance is given to
detecting smaller defects. The general approach in the industry is
to increase the imaging resolution by, for example, reducing the
wavelength of the light source or replacing light with other
radiation source.
However, when the upper scanned layer is comprised of high aspect
ratio (HAR) structures, the defect detection becomes problematic. A
typical defect that is difficult to detect, if impossible, is
illustrated in FIG. 1. Numerals 10, 11 and 12 indicate respectively
the metal layer, the dielectric layer, and the contact holes or
"vias" in layer 11. The vias are formed in the etch process. When a
contact hole or via 12 is filled with another metal to form the
electrical connection between two metal layers, this connection
should be well closed mechanically and electrically. In FIG. 1 it
is seen that, after the etch process two contact or vias are
completely obstructed or closed by non-conductive matter 13 and 14,
and another via is partially obstructed or closed by non-conductive
matter 15. The non-conductive matter may be polymer or residue
dielectric material that has not been successfully etched (removed)
or particles that have deposited on the wafer in the course of its
processing. Such defects are called partially opened contacts/vias.
Moreover, sometimes it happens that the process failed to create
the contact holes entirely, e.g.,. due to problems with the mask or
the stepper. Such a defect is called a "missing" contact hole.
Defects, such as partials (partially opened contacts/vias) and
missing contact holes are crucial for the final yield of the FAB,
because such defects are generally "killer" defects, Le., they make
the dies inoperative. With the current technologies of optical
scanning, it is difficult to detect defects on the bottom of the
vias or contacts, because of the following reason. The aspect ratio
of contacts and vias, viz. the ratio of their height to their
diameter, with today's technology is in the order of 5:1 to 10:1.
The vias or contact diameter is about 0.25 microns and is going to
decrease with the development of the technology. The wavelength of
the light used for inspection illumination with today's technology
is about 0.5 microns. Under such conditions, the light cannot reach
the bottom of the vias or contacts, and thus, the defects there are
not seen. The main reason is that the structure of the hole, having
a higher dielectric constant insulator surrounding a HAR hole of
low (i.e., air) dielectric constant acts as an "anti-waveguide" to
repel the light from the contact hole. This phenomenon behaves much
the same as a leaky mode in a waveguide.
There is, therefore, a need for a means that will permit the
detection of defects not revealed by optical scanning according to
the present art, and, in particular, the detection of the total or
partial obstruction of contact holes by non-conductive matter.
Other defects of interest for the present invention are, for
example, residual metal defects. These defects are of particular
interest since they can short out the circuit. Such defects can
show up especially in metal deposition and demacene processes,
depicted in FIGS. 1B and 1C respectively. In FIG. 1B, a metal layer
is deposited upon the insulator layer 100. Then, trenches (or other
structures) 130 are etched, thereby leaving only metal structures
110 upon the insulator 100. However, it may occur that some metal
residue 140 remains inside the trenches, and may cause a short. In
FIG. 1C, trenches (or other structures) are first formed in
insulator layer 105. Then, the trenches are filled with metal
(generally tantalum or copper) and the entire structure is polished
to remove any excess metal from the top surface of the insulator
105, thereby forming conductive structures 115 (such as bit lines).
However, some scratches may form on the top surface of the
insulator and be filled with the metal thereby creating a metal
residue defect 125, which can potentially short the circuit. Thus,
it is important to detect the presence of such metal residue
defects.
Another technology of interest for the present invention relates to
charged particle energy analyzer. Specifically, a specimen is
placed in the analyzer, and is bombarded with ionizing radiation so
as to dislodge charged particles from the specimen's surface. The
particles are then collected, and their energy spectrum is used to
determine the chemical constitution of the surface of the specimen.
With respect to this technology, the reader may refer to U.S. Pat.
Nos. 5,185,524 and 5,286,974.
A well known technique to release charged particles, i.e.,
electrons from a material's surface is by illumination of the
surface. This field is sometimes referred to a photo emission
electron microscopy (PEEM) and has previously been used to observe
samples. For an example of a PEEM for use with biological samples
the reader is referred to U.S. Pat. No. 5,563,411 to Kawata et al.
For use of PEEM in analysis of semiconductor structure the reader
is referred to "Characterization of p-n Junctions and
Surface_states on Silicon Devices by Photoemission Electron
Microscopy" M.Giesen et al., Appl. Phys. A 64, 432-430, 1997.
More relevant to the present invention is the following discussion
of the physics behind photo-emission. When the energy of a photon
E=hc/.lambda. that impinges on the material is larger than its
working function, i.e. .phi.<hc/.lambda., there is a probability
P that an electron will be released from the surface. Here, E is
the kinetic energy of a photon, h is the Plank constant, and c is
the speed of light.
The working function of any material represents the amount of
energy that one should apply in order to release an electron from
the material surface. Usually, the working functions of metals are
much lower than those of insulators, because the amount of free
electrons in metals is larger than in insulators. The working
functions of some commonly used metals in the semiconductor
industry and their corresponding wavelengths are summarized in the
table below.
Wavelength Metal .phi.(eV) (nm) Al 4.28 289 Cu 4.65 266 Ti 4.33 285
W 4.55 270
As can be seen from the table, a wavelength of less than 266 nm,
e.g., deep UV illumination, is required in order to release
electrons from these metals.
Another technology of interest for the present invention relates to
inspection of specimen using x-rays. Generally, an electron beam is
caused to impinge upon a metal, such as aluminum, to generate
x-rays.
The emitted x-ray is formed into a beam and caused to impinge upon
the specimen, thus causing emission of photoelectrons. The
photoelectrons are collected and an analyzer is used to determine
the chemical species on the surface of the specimen. Such
technology is described, for example, in U.S. Pat. Nos. 5,444,242
and 5,602,899 both to Larson and both assigned to Physical
Electronics, Inc. Physical Electronics' marketing literature (see
Physical Electronics document No. 9801 authored by Dan Hook)
describes the use of such technology for inspecting wafers to
determine the presence of photoresist residue. However, as can be
understood from the cited patents and literature, such system is
cumbersome and may not be readily implemented for "in-line"
inspection of wafers, especially as far as tilting the wafers is
concerned. Also, the throughput is limited due to the relatively
low intensity of the X-ray beam that is produced with the aid of an
energetic electron beam that impinge upon an Aluminum specimen.
SUMMARY OF THE INVENTION
The present invention provides for a method and apparatus for
inspecting HAR contact holes and vias.
The method of the invention is also advantageously applicable to
the detection of non-conductive or conductive foreign matter at any
location on the wafer.
It is a purpose of the invention to provide a method and an
apparatus that are quickly operable and provide high
throughput.
It is another purpose to provide a method and an apparatus that are
compatible, and even combinable, with optical inspection
methods.
Other purposes and advantages of the invention will appear as the
description proceeds.
The solution, provided by the invention, to the problem of the
detection of the total or partial obstruction of contact holes by
non-conductive matter, is based on the idea of material
classification rather than conventional imaging or scattered light
detection. Basically, therefore, the method of the invention
comprises the step of verifying the type of material at any
location on the wafer, and particularly at the bottom of the vias
or contacts. This same method can also be advantageously used to
detect other defects, such as metal residue defects.
Normally, at the bottom of a via or contact, there is metal, and
the surrounding is an insulator. One of the physical parameters
that distinguishes metals from insulators is their working function
(.phi.). The method of the invention, therefore, comprises, in its
preferred form, the step of determining whether the material at the
bottom of the vias or contacts is a metal or an insulator by
distinguishing between them according to their working
function.
The method according to the invention, comprises irradiating a spot
of the wafer surface with a beam having a wavelength sufficiently
shorter than the working function of the metal, such as deep UV
beam, collecting the electrons released by the irradiated wafer,
generating an electrical signal that is a function of the collected
electrons, and inspecting the signal to determine whether any
non-conductive material is present on said spot of the wafer, and
particularly, if said spot comprises contact holes or vias, whether
the contact holes or vias within the irradiated wafer spot are
open. The decision may be reached by comparing the electrical
signals measured in adjacent dies, or, if the dies have a periodic
structure, by comparing the signals obtained from adjacent cells
within the die.
The invention also provides an apparatus for carrying out the
method of the invention. The apparatus comprises a vacuum chamber
having therein a stage and chuck for supporting the wafer. An
illumination source generates irradiating energy which is formed
into a beam using appropriate optics so as to obtain the desired
beam spot of the wafers surface. An electron detector collects
electrons released from the wafer surface and sends a corresponding
signal to a processor for processing the signal to determine
whether the metal in said beam spot, and particularly at the bottom
of any hole within said spot, is exposed.
Preferably the electron detector comprises at least one electrode
mounted above the wafer and having a positive potential with
respect to the wafer.
The invention further provides for novel apparatus--hereinafter
called, "hybrid optical photo-electron wafer inspection apparatus",
or, briefly, "hybrid apparatus"--which enables concurrent optical
inspection of the wafer and electron detection for HAR opening
verification.
BRIEF DESCRIPTION OF THE DRAWING
The various advantages and features of the invention can be better
understood from the detailed description provided below, wherein a
reference is made to the accompanying drawings, in which:
FIG. 1A schematically illustrates opened and closed contact holes
in a wafer;
FIG. 1B schematically illustrates residual metal defects occurring
in the metal deposition process;
FIG. 1C schematically illustrates residual metal defects occurring
in the damacene process;
FIG. 2 schematically illustrates an apparatus according to an
embodiment of the invention; and
FIG. 3 schematically illustrates an apparatus according to another
embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiments hereinafter described relate to the
determination of whether a high aspect ratio contact hole or via is
open. However, as hereinbefore stated, the invention is not limited
to such embodiments or in general to the determination of the
presence of non-conductive matter in holes or vias, but includes
its determination at any location on the wafer. In the embodiments
hereinafter described by way of illustration the wafer is
irradiated with light of short wavelength to cause release of
electrons from the wafer. The electrons are collected to generate a
signal, and the signal is analyzed to determine whether the contact
holes or vias are open.
While the preferred embodiments described herein refer to a DUV
laser light, it should be appreciated that other short wavelength
light sources can be used. Specifically, laser light has been
generally preferred for scanning (e.g., laser printers, CD players,
etc.) because its spatial coherence permits creating a small beam
spot on the substrate. Notably, if one uses a spatially incoherent
beam from a source other than a laser, it is hard to obtain a small
and intense beam spot. However, since the beam spot diameter is
proportional to the wavelength, according to the invention, one can
choose an incoherent beam having sufficiently short wavelength and
obtain a desired beam diameter.
Thus, the invention will be described hereinafter with reference to
the use of a deep UV light beam, but this is not to be construed as
a limitation, and it should be understood that what is said with
reference to the illumination of a spot of the wafer surface with a
deep UV light beam, equally applies to the irradiation of said spot
with other light sources.
The illumination or irradiation of the wafer surface and the
collection of the released electrons should be carried out under a
vacuum.
In FIG. 2, numeral 20 generally designates a vacuum chamber and 21
is the wafer under inspection. An X-Y stage 35 is provided for
supporting the wafer 21 in chamber 20, although it should be
appreciated that other stages, such as a turntable, can be used for
scanning the wafer. Alternatively, the stage can be stationary and
the optical head can be scanned over the entire wafer.
Element 22 is a short wavelength light source and 23 indicates a
scanner for shifting the light beam 24 to scan the wafer
strip-wise. The light beam 24 is focused by objective 25 to form a
spot on the wafer surface. Vacuum chamber 20 also contains an
electron detector shown at 27, which has an opening for the passage
of the light beam issuing from objective 25. Numeral 40
schematically indicates an optional biased electrode.
As shown in FIG. 2, it is preferable to have the light source 22
outside the vacuum chamber 20, and to allow the beam 24 to enter
the chamber via window 28. Then the beam can be made to impinge
upon the scanner 23. Alternatively, the scanner may also be
situated outside the vacuum chamber 20, and the scanned beam can be
made to enter the chamber 20 via a window.
In operation, the beam is made to scan the wafer covering strips
having a width determined by the scanning angle of the scanner 23
and length determined by the travel of the stage in a first
direction, say in the Y direction. Once the beam completes scanning
one strip, the stage moves in the second direction, say the X
direction, an amount somewhat smaller than the strip's width, to
start scanning a second strip. The motion in the X direction is
made smaller than the width of the strip to allow for some overlap.
Of course, if a turntable is provided, the scanning would be in the
r-theta coordinates.
As the light beam impinges upon metal at the bottom of a contact
hole or via, electrons are emitted and detected by the detector 27.
On the other hand, if the contact hole or via is fully blocked, the
energy of the impinging light beam would be insufficient to cause
electron emission, SO that an alarm can be issued that the contact
hole is blocked or missing. Similarly, when the contact hole is
partially blocked fewer electrons will be emitted, and a
corresponding signal would be provided by the detector 27 so that
an alarm may be issued. The investigation of the signal can be done
using methods such as a threshold, or any suitable algorithm for
die-to-die, or a cell-to-cell comparisons, etc.
In a similar manner, when inspecting the substrate for metal
residue or similar defects, the wafer can be scanned to locate
electron emission from areas where there should be only an
insulator. For example, when the scan is performed in the trenches
130 of FIG. 1B or on the insulator part 105 of FIG. 1C, no
electrons should be emitted. If electrons are emitted, it signals
that there's metal residue inside on an insulating layer and a
defect alarm should be issued.
The illumination or irradiated spot may have an area that contains
a number of contact/vias at a time, and therefore the detected
photo electrons, as well as the scattered light, may belong to all
of them. Therefore, as provided by the Nyquist theorem, when a
defect is detected, its location is known with an accuracy that is
limited by the sampling rate of the detector's signal sampling. In
the preferred embodiment, the sampling rate can be adjust so that
the defect can be traced to nearly one of several holes which are
within the field of view of a review tool. That is, according to
the preferred embodiment, there is no need to have a high sampling
rate so as to detect the exact location of the plugged or
obstructed hole. Instead, the sampling rate is chosen so that the
plugged hole can be identified as one of several holes which would
appear in a field of view of the review tool, since when such a
defect is detected the wafer will be moved to a review tool for
further investigation. Therefore, all that needed is to identify
for the review tool a general location'so that the exact location
can be found within the field of view.
FIG. 2 also shows an optional biased electrode 40 which provides
two advantages. First, by biasing the electrode with respect to the
wafer, it helps pull the electrons from the bottom of the contact
holes. Specifically, since the invention is aimed at HAR holes,
electrons emitted from the bottom of the hole may hit the walls of
the hole and, therefore, not exit the hole. By applying a field
over the wafer, the electrons will be attracted vertically and exit
the hole easily. Second, in order to achieve a high throughput, it
is important to use high speed scanning. Consequently, it is
necessary to collect the electrons quickly. By applying the field
over the wafer, the emitted electrons are caused to accelerate
towards the detector, thereby shortening the time it takes to
collect the electrons. Also, it is important to keep the wafer
(specimen) neutral during the inspection process. This can be done
by ground connecting the wafer through the chuck.
In some cases, the conductivity of the wafer might be poor.
Consequently, when the electrons are pulled from the wafer, a
positive potential may be left on the wafer. This phenomenon is
known in the art as "charging effect." In order to avoid this
charging effect, an electron gun numbered (not shown) is optionally
placed in the vacuum chamber in order to direct electrons to areas
that are already scanned in order to keep the wafer neutral.
A hybrid defect inspection/HAR inspection system is depicted in
FIG. 3. Much of the elements depicted in FIG. 3 are similar to
those shown in FIG. 2 and, therefore, these elements are identified
with the same numerals. The system of FIG. 3 takes advantage of the
fact that, while usually sources of shorter wavelength light
produce smaller intensity, scattering of the light beam by defects
is increased as a function of 1/.lambda..sup.n, where .lambda. is
the illumination wavelength. Therefore, the system of FIG. 3 uses
scattering of the light for defect detection. For that purpose,
preferably four detectors, (only two detectors are shown, indicated
as elements 45) are arranged around the beam so as to detect any
light scattered from the wafer. Such an arrangement is generally
known as dark field arrangement, as used, for example, in the WF
736 inspection systems available from Applied Materials of Santa
Clara, Calif., and described in the above-cited U.S. Pat. No.
5,699,447.
An optional feature which can be implemented in either systems of
FIGS. 2 and 3 is exemplified in FIG. 2. Specifically, a "bright
field" detector 36 can be added in addition to the dark field
detector. In FIG. 2, a dichroic mirror 30 is used to deflect the
bright field reflection light towards the bright field detector 36.
Of courses other optics can be used to direct the reflected light
towards the bright field detector.
As can be understood, the invention as exemplified by the preferred
embodiment of FIG. 3 is advantageous in that it enables
simultaneous optical inspection of the wafer for defect and
electron detection for inspection of HAR contact holes and vias.
That is, when the beam 24 impinges upon the wafer the electron
sensor 27 can be used to detect any electrons emitted from the
wafer--thereby indicating that a contact hole is unobstructed--and
the light detectors can be used to detect any scattered
light--thereby indicating a defect on the wafer's surface (as is
known in dark field wafer inspection).
Another advantage of the hybrid system of FIG. 3 is that it enables
"on-the-fly" classification of certain metal defects. That is, when
a metal defect, such as the metal residue defects shown in FIGS. 1B
and 1C, are present, the light scattering system will sense the
defect and provide an alarm, while the electron collection system
will enable indication of whether the defect is made of metal. This
can help in distinguishing between washing stains, "empty"
scratches, metal residue, etc., thereby assisting in the
classification of the detected defects.
While specific embodiments of the invention have been described for
the purpose of illustration, it will be understood that the
invention may be carried into practice by skilled persons with many
modifications, variations and adaptations, without departing from
its spirit or exceeding the scope of the claims.
* * * * *